(1) Field of the Invention
The present invention relates to a method for controlling the length of an electrical arc in an arc generating machine such as an arc furnace, a gas-tungsten arc welding machine, with or without pulsed current (TIG machines), a gas-metal arc welding machine (GMAW machines) or an arc lamp with or without additional metal wire feeding.
The present invention also relates to a device for practicing the above-mentioned method.
(2) Description of Related Art
In the field of arc furnaces and arc welding machines, it is known that the electrical power fed to the arc column is a function of the length of the arc. Accordingly, in order to obtain good results when using an arc operating machine such as an arc furnace, an arc welder or an arc lamp, it is compulsory to adjust and control the distance separating the elements of the machine between which the arc forms. This control is particularly essential when one or both of these elements are eroded during operation of the machine.
In the particular case of an arc furnace, it is compulsory to maintain the length of the arc at its optimal operating value by maintaining the distance between the electrode(s) and the metal bath constant especially when the electrode(s) is or are mobile and have to be moved to compensate for the erosion of their tips, in order to keep as constant as possible the need of electrical power necessary for supplying the arc, to reduce as much as possible the erosion of the tips of the electrodes and to reduce as much as possible the erosion of the refractory material of the furnace.
Similarly, in the case of an arc welder, it is compulsory to control in a permanent manner the distance between the electrodes or the electrode and the metal part to be welded since the quality and regularity of the welding depends directly on this distance.
In order to ensure this necessary control of the length of the electrical arc in operating machines it has been proposed up to now to measure the arc voltage drop, since it is known that its value is a function of the length of the arc, and to monitor at least one of the electrodes of the arc operating machine to keep this value constant.
For measuring the arc voltage drop, an electrical voltage probe is usually connected to the power supply of the arc in the vicinity of the ends of the arc by means of wires. This method which thus involves the creation of a measuring loop in the power supply line for the arc, has several major drawbacks. Among these drawbacks, it should first be mentioned that the electrical voltage probe is never electrically insulated from the arc power supply. This results in very sharp problems when the arc generating machine is operating at high voltage or when the arc supplying current is floating. As another major drawback, it should also be mentioned that the measuring loop is subject to parasitic voltages induced in the loop in particular when the arc current undergoes high variation.
Another major drawback of the conventional method used up to now is the difficulty of obtaining an exact value of the arc voltage drop, as the value obtained with the measuring loop is, in fact, the sum of the following voltage drops:
(1) the actual, arc column voltage drop;
(2) the anode and cathode voltage drop;
(3) the ohmic drop in the electrode and in the part of the supply circuit included in the measurement loop;
(4) the induced voltages due to mutual inductance between the phases of the current in a three phase current supply circuit when, for example, the measuring loop is used in an industrial arc furnace; and
(5) the parasitic voltages induced in the measurement loop.
All these voltage drops, except the arc column voltage drop which is a function of the arc length, are very difficult to measure and/or determine and therefore it is almost impossible to isolate the arc column voltage drop from the value obtained with the measuring loop since this value is often a small fraction of the total measured voltage.
In the particular field of arc furnaces and arc welding machines, it is also known that the arc formed between the electrode elements of the machine produces an audible, acoustical signal when the machine is supplied with an AC or modulated current. It is also known that the amplitude of the acoustical signal generated by the arc is proportional to the time derivative of the power fed to the arc column.
From this theoretical knowledge, it has already been proposed to use the detected value of the amplitude of the acoustical signal generated by the arc for controlling some parameters of an arc-welding operation.
By way of example, British Pat. No. 1,430,824 published on Apr. 4, 1976, in the name of the Japanese firm Mitsui Ship Building and Engineering Co. Ltd. discloses a method for controlling the operation of an arc welding machine by measuring the intensity of the sound generated by the arc, detecting any abnormal variation in the intensity of the so measured sound and acting on some welding characteristics of the welding machine in response to the so detected abnormal sound. The parameters mentioned in this British Patent as being possibly controllable by measuring the intenstiy of the acoustical signal generated by the arc column are the welding speed, that is the speed at which the electrode moves in translation with respect to the piece of metal to be welded, the plasma gas flow when use is made of a plasma welding machine, the intensity of the welding current and the metal wire feed speed.
This British Patent which is concerned much more with a very broad, theoretical principle than with a practical embodiment of this principle, does not mention that use can be made of the acoustical signal generated by the arc for controlling the length of this arc, although this is an essential condition for obtaining a good weld.
This patent does not mention either that use can also be made of the acoustical signal generated by the arc column for controlling the operation of a continuous or three-phase current arc furnace. On the contrary, this patent makes reference only to arc welding machines or lamps.
Last of all, it should be noted that this British Patent suggests the detection of the intensity of the sound generated by the arc column only for detecting and correcting the anomalies of operation occurring during the welding operation. This patent does not mention that use can be made of the sound generated by the arc column for controlling the arc generating machine in a permanent and positive manner.